1. Field of the Invention
The invention relates to the field of technical ceramic and relates to transparent polycrystalline sintered ceramics of cubic crystal structure for applications with increased mechanical stress, e.g., as protective or armoring ceramic.
2. Discussion of Background Information
The quantity responsible for the transparency of materials is the real in-line transmission (RIT) to be measured from the detected intensity only with a very narrow aperture angle of about 0.5° for the purpose of excluding scattered light.
In the case of lacking or extremely low light absorption, the transmission for optically homogeneous materials such as glass or for monocrystals is limited only by the material-specific reflection Rs=((n−1)/(n+1))2 at the front and back side, respectively, determined by the refractive index n. The resulting theoretical maximum value of the transmission Tmax is Tmax=(1−R) where R=2 Rs/(1+Rs) for highly transparent substances, taking into consideration the multiple reflection, or Tmax=(1−Rs)2 for materials of low transparency, i.e., negligible multiple reflection; Tmax, e.g., for colorless monocrystals such as Al2O3 (corundum, sapphire; n=1.760 or 1.768) is about 85.8%, for MgO.Al2O3 spinel (n=1.712-1.736) about 86.9%, for Y2O3 (n=1.78-1.79) about 85.3%, and for Y—Al garnet (n=1.833) about 84.1%. For ZrO2 (n=1.98-2.2), this limit can vary within a broader range between about 75 and 81%, depending on the composition.
With light transmission through the structure of sintered polycrystalline ceramics, on the other hand, RIT is usually further reduced by the following processes:
1. diffuse scatter at pores (depending on the size and number of the pores), and
2. especially in non-cubic ceramics such as corundum (α-Al2O3), additional light scatter through double refraction at each transition of the light ray from one into the next crystallite of the structure.
The scatter losses must therefore be kept low in all sintered ceramics through the lowest possible residual porosity and through pore sizes that, if possible, are smaller than the wavelength of the light.
Since only this first-named scatter mechanism occurs in cubic sintered ceramics, their transparency is thus not subject to any direct influence of grain size. On the other hand, in particular in materials of non-cubic crystal structure, the second scatter contribution must be reduced either by extreme grain growth (i.e., approaching the monocrystalline state) or by extremely small structure grain sizes. For nanocrystalline structures (e.g., tetragonal ZrO2 with grain sizes around or below 60 nm (V. Srdic et al., J. Am. Ceram. Soc. (2000), 1853-1860), as is well known a high transparency is ultimately given independent of the degree of tetragonal distortion if the manufacturing process is designed so that even the last hundredths of a percent of residual porosity are avoided.
This situation, which differs depending on the crystal structure, explains why no RIT values >70% could be achieved hitherto for the most advanced (finest-grained, low-pore) sinter corundum (trigonal) at thicknesses about or above 1 mm, whereas, e.g., for MgO.Al2O3 spinel (cubic), measured values of about 80% and thus close to the theoretical maximum were reported. As a particular advantage of very high RIT, it must also be noted that the influence of thickness disappears on approaching the theoretical maximum, whereas vice versa any greater scatter losses naturally increase with the thickness of the light-scattering material and the transparency is then only guaranteed for very thin components. Thus, such an influence of thickness is a clear criterion for the presence of considerable light scatter—i.e., low transparency—and for the origination of any high transmission measured values from a too large aperture angle that makes it impossible to measure the real in-line transmission.
Because of this superiority of cubic materials, attempts were already undertaken early to produce hot-pressed transparent ceramics from such substances, for the purpose of optimizing the mechanical properties at first combined also with the aim of the lowest possible grain growth during the sintering. Examples are MgO.Al2O3 spinel ceramics with average grain size of about 1 μm (U.S. Pat. No. 3,767,745) and Y—Al garnet ceramics with a grain size of about 5 μm (U.S. Pat. No. 4,841,195). However, none of these developments led to satisfactory transparency values, due to a certain residual porosity in the grain boundaries of these fine-grained structures: U.S. Pat. No. 3,767,745 gives no data for quantifying the degree of transparency of the spinel achieved, while for the above-mentioned garnet a transmission was measured at 589 nm wavelength in spite of a very large aperture angle of 8° (0.14 rad), which transmission only reached max. 57% for, e.g., 0.8 mm-thick plates, which corresponds to just 65% of the theoretical maximum value (U.S. Pat. No. 4,841,195). For fine-grained spinel ceramics with grain sizes of between 2 and 5 μm, such large (unfortunately usually unstated) aperture angles of the light measurement have even led to measuring results of up to 80%, but unlike the real in-line transmission explained above, they do not disclose any actual transparency: in the example of EP 334 760 A1, this becomes clear from the reported considerable influence of thickness on the visible transmission of the sintered spinel, so that to determine the theoretical upper limit correctly, the above-mentioned equation Tmax=(1−Rs)2 was used for materials of low transparency (without multiple reflection).
As a result, therefore, the special advantage of cubic transparent sintered ceramics of being able to achieve high transmission values generally even with coarser structures, has lead to development being given priority in a direction where attempts are made to eliminate the residual porosity by particularly high sintering temperatures, even when nanocrystalline powder raw materials are being used—while accepting strong grain growth. In this context, production temperatures of 1700° C. for Y2O3— and Y—Al garnet ceramics are then called “low temperature fabrication,” and the ceramics of optimized transparency produced finally exhibit coarse structures with average grain sizes of about 20 μm (N. Saito et al., J. Am. Ceram. Soc. (1998), 2023-2028; J.-G. Li et al., J. Am. Ceram. Soc. (2000), 961-963), in spite of the use of nanopowders (average particle sizes about 70-100 nm). As a result of such extreme grain growth, pores are frequently enclosed in the growing crystallites intragranularly, so that for 1 mm-thick plates—with unspecified quite large aperture angle—the in-line transmission measured at a wavelength of, e.g., 600-650 nm achieves only 42% of the theoretically possible maximum value; thus this transmission, in spite of the advantage of the cubic crystal structure, actually lags behind the values achieved as RIT of about 60% (=70% of the theoretical maximum value) under otherwise similar measuring conditions for trigonal sinter corundum in spite of its double refraction (A. Krell et al., J. Am. Ceram. Soc. (2003) 12-18). Similarly, for cubic Y-stabilized ZrO2, which was doped with titanium to promote the grain growth, the highest transmission is achieved for the most coarse-grained structure of about 150 μm (here by the combination of sintering at 1700° C. with hot isostatic redensification [HIP] at 1500° C.), whereby this achieved maximum value of a (likewise unspecified) transmission through a plate with a thickness of 0.73 mm is only 64% (K. Tsukuma et al., Advances in Ceramics, Vol. 24 [Zirconia III, 1988], 287-291).
As for the rest, the latter result for transparent ZrO2 ceramic shows that limits are set on the actually achievable transmission even for conventional cubic sintered ceramic of ZrO2, these limits being of the type that vice versa similar transparency results should also be achievable with tetragonal ZrO2, as long as the c/a axial ratio of the elementary cell deviates by not more than about 1.5% from value 1 (=cubic), and therefore no considerable deviation occurs from the approximately cubic (optically isotropic) behavior.
Nor have alternative sintering processes led hitherto to the development of fine-grained cubic transparent ceramics. Thus, for a cubic AlON ceramic produced by microwave sintering, the highest transmission was produced precisely with the highest sintering temperature (1800° C.), which led to an average grain size of 40-50 μm and limited the realized light transmission in the visible range to low values of about 43%, even at a sample thickness of only 0.6 mm (D. Agrawal et al., Ceramic Trans. Vol. 134 (2002), 587-593).
With MgO.Al2O3 spinel, although high transmission values were achieved, RIT values in the visible range were about 60% for 2 mm-thick plates after the combined use of hot pressing (1400-1500° C.) with HIP at 1900° C. (A. F. Dericioglu et al., J. Europ. Ceram. Soc. (2003) 951-959) or, with the addition of 1.5% of Li doping, also a transmission close to the theoretical limit after hot pressing at 1410° C. followed by HIP at 1500° C. (EP 447390 B1), but even for these ceramics the average structure grain size in the first example was always >150 μm, and in the second example it is given as less than 150 μm, starting from a powder described as “submicron.”
These coarse structures reduce even further the mechanical characteristics of the cubic ceramics, which are already at a substance-related (as a monocrystal) disadvantage compared with the trigonal corundum, and lead to lower strengths and readier dislocation activity with low hardness. Thus, e.g., the microhardness (test load 200 g) of transparent AlON ceramic with 150 μm grain size is only 13.8 GPa, that of transparent spinel with bimodal grain size distribution of between 10-20 and 100-200 μm is as low as 12.1 GPa (J. J. Swab et al., Ceramic Trans. Vol. 122 [0015], 489-508), which must be compared with the hardness values of the same measurement for transparent sinter corundum of >20 GPa and is of particular significance for its use as a protective ceramic: systematic testing with sinter corundum of graduated grain sizes and hardness have shown that the mass effectiveness of the ceramics characterizing the anti-ballistic protective effect declines by about ⅓ with a hardness dropping from 21 to 12 GPa (A. Krell et al., Ceramic Trans. Vol. 134 [0016], 463-471). The coarse-grained cubic sintered ceramics of low hardness known hitherto therefore require thicker components for an equivalent protective effect, which is accompanied by the disadvantage of higher weight.
The disclosures of each the above-cited documents are incorporated by reference herein in their entireties.